The invention relates to a device for ventilating and aerating a fuel tank.
Devices of the aforementioned type are provided between the fuel tank and an activated carbon filter to prevent an undesired leaking of volatile hydrocarbons from the fuel tank into the environment of the motor vehicle. The tank shut-off valve is a controllable valve, for example an electromagnetic valve, which is normally closed and is opened during refueling of the fuel tank to conduct a gas mixture which was displaced from the fuel tank, through the activated carbon filter and to adsorb the hydrocarbons (HC) contained in the gas mixture so that only purified air is released into the environment. The two tank pressure control valves are an overpressure valve and a negative pressure valve, which are also normally closed. The overpressure valve opens automatically when for example in the case of rising ambient temperatures or during a residual heating phase after turning off the internal combustion engine a predetermined overpressure forms in the interior of the fuel tank as a result of evaporation of fuel, while the negative pressure valve opens automatically when in the case of falling ambient temperatures fuel condenses in the interior of the fuel tank and as a result a predetermined negative pressure forms in the interior of the fuel tank.
Known devices of this type are usually configured as combination valve in which the tank shut-off valve and the two tank pressure control valves which are configured as bypass valves, are combined, wherein the tank pressure control valves in praxis usually have a single membrane, which in the case of overpressure or negative pressure is deflected against the force of a spring to open the valve. The opening pressure of the valve is determined by the surface area of the membrane and the spring characteristic of the spring which acts on the membrane and can therefore be influenced only by changing the spring characteristic when a single membrane is used. For example, a harder spring can be used when the opening pressure of the valve in the case of overpressure in the tank, i.e. the positive pressure difference between the interior of the tank and the interior of the activated carbon filter, at which the valve opens, is to be set to a greater value. This has the consequence however, that the valve in the case of a negative pressure in the fuel tank also only opens in the case of a greater pressure differential between the interior of the fuel tank and the interior of the activated carbon filter, i.e. when a greater negative pressure is present in the interior of the fuel tank. This does not allow for adjusting the opening pressures for the both cases of an overpressure or a negative pressure in the fuel tank, respectively in a sufficiently flexible manner.